The present invention is directed to a method and apparatus for centrifugal casting of metal articles. More particularly, the invention is directed to a method and apparatus for forming a mold lining of a refractory material in the centrifugal casting of metal articles.
Centrifugal casting is a common method used for casting tubular metal articles including engine cylinder liners. The centrifugal casting apparatus is typically a cylindrical shaped metal mold that is rotated about the longitudinal axis at sufficient speed to distribute the molten metal along the inner surface of the mold. The molds are generally made of metal and have the inner mold surface covered with a lining material to protect the mold from damage and overheating by contact with molten metal. The lining material also is provided to prevent the molded article from bonding to the mold surface.
One method for applying a lining to centrifugal casting molds applies a slurry of a fine particulate refractory material. The refractory materials are typically zircon powder or silica powder and a binder such as bentonite clay. Applying a slurry of a refractory material to a mold surface has exhibited some success. However, a disadvantage of this method is that the mold requires adequate venting to vent water vapor produced during the casting process.
Other methods of forming a mold lining use a binder material, such as a resin, to bond the particles together and to bond the material to the surface of the mold. These methods can be difficult to apply and form a uniform surface. In addition, the application of a lining material using a binder can be expensive and produce gaseous products by the heat from the molten metal during the casting of the metal article.
Another method for forming a lining in a centrifugal casting mold is disclosed in U.S. Pat. Nos. 4,124,056 and 4,260,009 to Noble. The lining material disclosed in this patent is a dry, binderless particulate refractory material that is applied to the surface of the rotating mold. As disclosed in this patent, the refractory material is introduced into the rotating mold as a dry powder without the use of binders or slurries. The method of this patent is primarily directed to forming a straight or contoured surface on the lining for molding cylindrical articles. The refractory material is subjected to centrifugal force to form the mold lining. A blade removes the excess amount of the mold lining. However, it has been found that the process of this patent does not form a sufficiently uniform or hard molding surface and that imperfections can form in the molded article, because the refractory grains are not compacted and densified.
To overcome the disadvantages of the process of the above-noted U.S. patents, a vibration inducting device was coupled to the mold to vibrate the mold as the mold rotates. The method of vibrating the mold while the refractory material is applied to the inner surface of the rotating mold is disclosed in U.S. Pat. No. 4,632,168 to Noble. However, repeated trials have found that vibrating the mold does not produce a satisfactory lining for producing a molded article because the angular grains of milled refractory are held in place by centrifugal force, and will not move and pack when vibrated as would round grains. The resulting lining is not of a uniform density and has soft spots that result in imperfections in the molded article.
U.S. Pat. No. 4,632,168 to Noble discloses a molding process where a porous primary layer and a facing layer forms a vented particulate refractory lining in the mold to vent air pressure through the primary layer caused by the elevated temperatures when the molten metal contacts the facing layer.
The prior methods of forming a lining from a refractory material in a centrifugal casting mold have met with a limited success. Linings formed from refractory materials that include a binder or are applied as a slurry require adequate venting of the mold to allow the escape of water vapor and gaseous components of the binder material. The prior method of forming a lining from a dry, binderless particulate refractory material typically produce a lining that is not of a uniform density throughout the lining that results in soft spots in the lining. These soft spots do not enable the production of a molded article having a uniform surface since the soft area in the lining is typically mirrored in the molded article. Accordingly, there is a continuing need in the industry for an improved method and apparatus for forming a lining material in a centrifugal casting mold.
The present invention is directed to a method and apparatus for centrifugally casting metal articles. The invention is further directed to a method and apparatus for forming a mold lining in a centrifugal casting mold.
Accordingly, a primary object of the invention is to provide a method and apparatus for forming a mold lining from a dry, binderless particulate refractory material without the use of binders or adhesives.
Another object of the invention is to provide a method and apparatus for forming a lining from a dry, binderless particulate refractory material having a smooth molding surface without imperfections that would otherwise be mirrored in the molded article.
A further object of the invention is to provide a method and apparatus for forming a mold lining in a centrifugal casting mold where the lining is compacted and densified to avoid the formation of soft spots in the mold lining.
Still another object of the invention is to provide a method and apparatus for forming a molding lining in a centrifugal casting mold from particles of a dry, binderless particulate refractory material where the particles are compacted by centrifugal force and mechanical force to form a densified mold surface.
Another object of the invention is to provide a method and apparatus for forming a mold lining in a centrifugal casting mold from compacted particles of a dry, binderless particulate refractory material where the lining is substantially impervious to air and molten metal.
Another object of the invention is to provide a method and apparatus for forming a mold lining in a centrifugal casting mold from particles of a dry, binderless particulate refractory material where the particles are sufficiently compacted and densified so that the voids between the particles are not interconnected with the voids between adjacent particles.
Another object of the invention is to provide a method and apparatus for forming a mold lining from particles of a dry, binderless refractory material where the particles are subjected to centrifugal force and mechanical redistribution and compaction to expel air between the particles of the lining.
Still another object of the invention is to provide a method and apparatus for mechanically compacting and redistributing particles of a dry, binderless refractory material on the inner surface of a rotating centrifugal casting mold to form a mold lining of interlocked particles.
Another object of the invention is to provide a method and apparatus for forming an air impervious layer of particles of a dry, binderless refractory material in a centrifugal casting mold where the particles are interlocked to form a hard mold lining.
Another object of the invention is to provide a method and apparatus for forming a lining in a centrifugal casting mold by mechanically redistributing and compacting particles of a dry, binderless particulate refractory material while subjecting the particles to the force of inertia of rotation combined with centrifugal force in a rotating mold.
A further object of the invention is to provide a mold lining in a centrifugal casting mold by dispersing particles of a dry, binderless refractory material on the inner surface of a rotating mold and contacting the particles with a tool to redistribute and compact the particles on the inner surface of the mold.
Another object of the invention is to provide a method and apparatus for venting a mold lining and removing heat from a centrifugal mold by using mold vents placed at different diameters on the mold to create air flow through a porous refractory material. The vents include an inlet and an outlet that are radially spaced apart with respect to an axis of rotation of the mold.
These and other objects of the invention are basically attained by providing an apparatus for producing molded articles by centrifugal casting. The apparatus comprises a centrifugal casting mold having an inner mold surface with a longitudinal length and a generally open axial end for receiving a molten metal, a mold-rotating device for rotating the centrifugal casting mold, a dispensing device for introducing a predetermined amount of particles of a dry, binderless milled refractory lining material on the inner mold surface of the centrifugal casting mold substantially uniformly along the longitudinal length of the mold to form a layer of the lining material on the inner mold surface, a plowing device for contacting the layer of the lining material while rotating the mold and redistributing, compacting, and densifying the refractory material in the mold to form a compacted and substantially air impervious mold lining of interlocking particles of the refractory material, and a molten metal supply device for supplying molten metal onto the mold lining in the mold.
The objects of the invention are further attained by providing a method for the centrifugal casting of metal articles. The method comprises the steps of providing a rotary mold assembly having an elongated cylindrical mold with an inner mold surface and a generally open axial end. A predetermined amount of particles of a dry, binderless milled refractory material is introduced onto the inner mold surface while continuously rotating the cylindrical mold at a sufficient speed while distributing the refractory material along the inner mold surface and forming a layer of the refractory material on the inner mold surface. The layer of the refractory material is contacted with a plowing device while continuously rotating the cylindrical mold to physically redistribute, compact and densify the particles of the layer of the refractory material and form a substantially air impervious mold lining. The mold is rotated at a speed sufficient to cast a molten metal against the inner surface of the mold, and introducing a molten metal onto the mold lining and molding the metal.
These and other objects, advantages and salient features of the invention will become apparent to one skilled in the art in view of the following detailed description of the invention annexed drawings which form a part of this original disclosure.